Polyhydroxycarboxylic acid and its production process

ABSTRACT

Polyhydroxycarhoxylic acids are provided, which are controlled in terms of the rate of biodegradability, and give molded or otherwise formed articles that are of uniform quality with neither premature strength drop nor premature deterioration of retention of outside shape, and their production process is provided as well. The poly-hydroxycarboxylic acids are obtained by ring-opening polymerization of cyclic esters. The polyhydroxy-carboxylic acids have a weight-average molecular weight (Mw) in the range of 10,000 to 1,000,000, a molecular weight distribution in the range of 1.0 to 2.5 as represented by the weight-average molecular weight-to-number-average molecular weight ratio (Mw/Mn) and a yellowness index (YI) of 40 or less, and have a precisely controlled rate of biodegradability.

TECHNICAL FIELD

The present invention relates generally to a polyhydroxycarboxylic acidobtained by the ring-opening polymerization of cyclic esters such asglycolide or lactide and having biodegradability and its productionprocess, and more specifically to a less colored polyhydroxycarboxylicacid having a controlled rate of biodegradation and its productionprocess. In particular, the present invention is concerned with apolygylcolic acid (i.e., polyglycolide) that is less colored andimproved in melt stability and its production process.

The polyhydroxycarboxylic acids of the present invention such aspolyglycolic acid and polylactic acid or their copolymers are useful forvarious molded or otherwise formed articles such as sheet, films andfibers, composite materials (e.g., multilayer films or containers), andso on.

BACKGROUND ART

The ring-opening polymerization of bimolecular cyclic esters ofhydroxycarboxylic acid (also called “cyclic dimmers”) may yieldpolyhydroxycarboxylic acids. Typical of such cyclic esters are glycolidethat is a bimolecular cyclic ester of glycolic acid and lactide that isa bimolecular cyclic ester of lactic acid. The ring-openingpolymerization of glycolide yields polyglycolic acid (i.e.,polyglycolide), and the ring-opening polymerization of lactide yieldspolylactic acid (i.e., polylactide).

Polyglycolic acid and polylactic acid obtained by the ring-openingpolymerization of cyclic esters or polyhydroxycarboxylic acids such asring-opened copolymers of lactide and glycolide have been known asbiodegradable polymer materials, and their application to surgicalsutures, etc. have been long proposed (for instance, U.S. Pat. Nos.3,297,033 and 3,636,956).

Polyglycolic acid in particular, because of being better in heatresistance, gas barrier properties, mechanical strength, etc. than otherbiodegradable polymer materials, is finding new applications to sheets,films, vessels, injection-molded articles and so on (Japanese PatentApplication Laid-open (A) Nos. 10-60136, 10-80990, 10-138371 and10-337772).

These polyhydroxycarboxylic acids have difficulty in controlling theirrate of biodegradation, although they are biodegradable andenvironmentally friendly polymer materials. So far, the rate ofbiodegradation of polyhydroxycarboxylic acids has generally been thoughtof as being dependent on their average molecular weight. The rate ofbiodegradation may be quantitatively determined to a certain degree byburying, for instance, a polyhydroxycarboxylic acid molded article inthe ground to observe the period of its disintegration. This method iscalled soil degradability test.

When polyhydroxycarboxylic acid molded articles are tested for theirdegradability in the ground, it has so far been considered that thehigher the weight-average molecular weight of polyhydroxycarboxylicacids, the longer the period of time needed for disintegration becomes,and the lower the weight-average molecular weight, the shorter theperiod of disintegration time becomes. It is understood that whenpolyhydroxycarboxylic acids have a very low weight-average molecularweight, their time of disintegration in the ground is generally short.

However, the results of the inventors' studies have indicated that therate of biodegradation of polyhydroxycarboxylic acids is not necessarilydependent on their average molecular weight such as weight-averagemolecular weight. The same holds true even when instead ofweight-average molecular weight, solution viscosity, melt viscosity andso on are used as the index to average molecular weight.

In general, when polyhydroxycarboxylic acids have a fast rate ofbiodegradation, they have some merits: biodegradation of used-uppolyhydroxycarboxylic acid molded articles and ease with which they canbe composted. However, such molded articles have limited applications tovery-short-time fields or low-strength fields.

When polyhydroxycarboxylic acid molded articles such as films orcontainers are used in application fields where durability and outsideshape retention on much the same order as in ordinary plastic moldedarticles are expected, too a fast rate of biodegradation causespremature drops of articles' strength, and makes it difficult to retainthe outside shape of articles over an extended period of time. Thus,there have been attempts to obtain molded articles improved indurability and outside shape retention without detriment to theirbiodegradability by allowing polyhydroxycarboxylic acids to have highermolecular weight.

Contrary to expectation, however, it has been found that only by use ofhigh-molecular-weight polyhydroxy-carboxylic acids, it is stilldifficult to keep hold of strength and outside shape while prematurebiodegradability is fully minimized. In addition, it is still difficultto make products of consistent quality because there are variations inthe rate of biodegradation for each polyhydroxycarboxylic acidproduction lot. On the other hand, polyglycolic acids obtained by thering-opening polymerization of glycolide are vulnerable to colorationupon polymerization at elevated polymerization temperatures for longperiods of time.

Thus, it is still difficult to control the rate of biodegradation ofpolyhydroxycarboxylic acids while their coloration is reduced, andanything significant about how to achieve this is not proposed at all.

Referring to the polyglycolic acid encompassed in polyhydroxycarboxylicacids, there is unavailable any well-established production technique asyet, and so it is still difficult to produce polyglycolic acid that canyield less colored molded articles.

Polyglycolic acid, when it is poor in melt stability, makes itimpossible to mold its melt in a stable manner. Polyglycolic acid, whenit is vulnerable to coloration, detracts from commercial value, andoffers hygienic problems as well. When polyglycolic acid has a fast rateof biodegradation, it is difficult to put the service life of productunder control although the polyglycolic acid can be easily composted.

U.S. Pat. No. 3,297,033 discloses that ring-opening polymerization iscarried out at 185 to 190° C. while glycolide mixed with apolymerization catalyst is charged into a glass tube, and that whitepolymers are obtained after cooling (Example 1). By carrying out thering-opening polymerization at temperatures lower than the melting point(about 220° C.) of polyglycolic acid, it is thus possible to obtain lesscolored polymers.

However, lower polymerization temperatures render the resulting polymerlikely to crystallize and solidify during polymerization reactions,whereby the polymerization reactions tend to become inhomogeneous. Theresulting polyglycolic acid is so poor in melt stability that whenextrusion molded into various articles such as sheets, films and fibers,it is difficult to carry out extrusion molding in a stable fashionbecause of large melt viscosity changes.

U.S. Pat. No. 3,468,853 discloses a process wherein glycolide mixed witha polymerization catalyst is subjected to ring-opening polymerization ata temperature of 205 to 235° C. until viscosity reaches a substantialequilibrium. However, long-term ring-opening polymerization at elevatedtemperatures often causes the resulting polyglycolic acid to be colored,greatly detracting from commercial value.

U.S. Pat. No. 2,668,162 discloses a polyglycolic acid production processwherein glycolide mixed with a polymerization catalyst is subjected toring-opening polymerization at 150 to 200° C. to produce alow-molecular-weight polymer, and the polymer is then heated to 220 to245° C. to increase its melt viscosity. With this process, however, itis difficult to prevention coloration of the resulting polyglycolic acidbecause a time-consuming heating step is needed and rapid heating tendsto lead to heating variations.

DISCLOSURE OF THE INVENTION

An object of the present invention is to provide polyhydroxycarboxylicacid that is controlled in terms of the rate of biodegradation andreduced in respect of coloration, and its production process.

Another object of the present invention is to providepolyhydroxycarboxylic acid that can yield a molded or otherwise formedarticle showing uniform quality with no premature drops of strength andoutside shape retention.

Yet another object of the present invention is to provide polyglycolicacid that is remarkably improved in terms of melt stability and, at thesame time, is reduced in respect of coloration, and its productionprocess.

A further object of the present invention is to provide polyglycolicacid that is excellent in melt stability, less colored, and ofcontrolled biodegradability, and its production process.

As a result of studies of why the rate of biodegradation ofpolyhydroxycarboxylic acid is not necessarily depending on its averagemolecular weight, the inventors have noted that conventionalpolyhydroxycarboxylic acids have not any well-controlled molecularweight distribution. When polyhydroxycarboxylic acid has a widemolecular weight distribution, it has a high weight-average molecularweight or melt viscosity on average; however, the part(low-molecular-weight part) of polyhydroxycarboxylic acid in alow-molecular-weight region is subjected to premature biodegradation,which may otherwise cause the whole strength of product to drop and theoutside shape of product to become worse. In addition, the presence of alarge amount of the low-molecular-weight part sensitive to prematurebiodegradation may cause the rate of biodegradation of the wholepolyhydroxycarboxylic acid molded product to become fast.

Therefore, the inventors have pursued studies, finding that apolyhydroxycarboxylic acid having a specific range of weight-averagemolecular weight and a relatively sharp molecular weight distributionshows practical strength properties with a controlled rate ofbiodegradation, and yields a molded article of uniform quality.

A narrow molecular weight distribution of polyhydroxycarboxylic acidimplies that it is subjected to substantially uniform biodegradationbecause of a reduced amount of the low-molecular-weight part susceptibleto biodegradation. Consequently, it is possible to obtainpolyhydroxycarboxylic acid that has controlled biodegradability andlimited variations in the rate of biodegradation. By regulating themolecular weight distribution to a narrow range and adjusting theweight-average molecular weight of polyhydroxycarboxylic acid, it ispossible to control the rate of biodegradation as desired.

On the other hand, it has also been found that when the polymerizationreaction is carried out at high polymerization temperatures for a longperiod of time for the adjustment of the molecular weight distributionof polyhydroxycarboxylic acid, the resulting polymer is sensitive tocoloration. At lower polymerization temperatures, however, the molecularweight distribution tends to become wide. With this in mind, after thepolymerization for polyhydroxycarboxylic acid was carried out, anadditional polymerization was performed at a temperature lower than thepolymerization temperature. As a result, it has been found thatpolyhydroxycarboxylic acid having a sharply controlled molecular weightdistribution can be obtained while its coloration is considerablyreduced. For this process, it is preferable that the firstpolymerization for polyhydroxycarboxylic acid should be carried out at arelatively high temperature for a relatively short time.

Furthermore, the inventors have found that polyglycolic acid having muchmore improved melt stability and a reduced yellowness index (YI) can beobtained by subjecting glycolide to ring-opening polymerization in amolten state, then converting the resulting polymer from the moltenstate to a solid state, and finally kneading the solid-state polymer ina molten state with the application thereto of heat. After conversion tothe solid state, it is acceptable to carry out a solid-phasepolymerization followed by kneading in a molten state.

In accordance with the process of the present invention, it is possibleto obtain polyglycolic acid having improved melt stability as expressedin terms of the retention of melt viscosity of at least 40%, and/or theyellowness index (YI) of up to 40 as measured using a sheet obtained bypress molding and crystallization of the polyglycolic acid. It is herenoted that the retention of melt viscosity is defined by the ratio ofthe viscosity (η60) measured after a 60-minute retention at 250° C. tothe initial viscosity (η₀) measured after a 5-minute preheating at 250°C. (i.e., (η₆₀/η₀)×100).

The polyglycolic acid having improved melt stability according to thepresent invention is excellent in melt stability upon melt molding, andso can provide molded or otherwise formed articles having improved colortone such as sheets, films and fibers. By regulating the weight-averagemolecular weight and molecular weight distribution of the polyglycolicacid, it is also possible to place the biodegradability of thepolyglycolic acid under control.

The present invention has been accomplished on the basis of thesefindings.

Thus, according to one aspect of the present invention there is provideda polyhydroxycarboxylic acid obtained by ring-opening polymerization ofa cyclic ester and controlled in terms of the rate of biodegradation,characterized in that said polyhydroxycarboxylic acid has:

-   -   (a) a weight-average molecular weight (Mw) in the range of        10,000 to 1,000,000,    -   (b) a molecular weight distribution in the range of 1.0 to 2.5        as defined by the weight-average molecular        weight-to-number-average molecular weight ratio (Mw/Mn), and    -   (c) a yellowness index (YI) of up to 40 as measured using a        sheet obtained by press molding and crystallization of said        polyhydroxycarboxylic acid.

According to another aspect of the present invention, there is provideda polyhydroxycarboxylic acid production process characterized bysubjecting a cyclic ester to ring-opening polymerization at apolymerization temperature of 120 to 250° C. for 3 minutes to 50 hours,and then to an additional polymerization for 1 to 50 hours at atemperature 10 to 50° C. lower than said polymerization temperature.

According to yet another aspect of the present invention, there isprovided a polyglycolic acid having melt stability characterized byhaving:

(I) a retention of melt viscosity of at least 40% as defined by theproportion of the viscosity (η60) of said polyglycolic acid measuredafter a 60-minute retention at 250° C. to the initial viscosity (η₀) ofsaid polyglycolic acid measured after a 5-minute preheating at 250° C.(η₆₀/η₀)×100), and/or

(II) a yellowness index (YI) of up to 40 as measured using a sheetobtained by press molding and crystallization of said polyglycolic acid.

According to a further aspect of the present invention, there isprovided a process for producing a polyglycolic acid having meltstability comprising steps of (1) subjecting glycolide to ring-openingpolymerization in a molten state, (2) converting the resulting polymerfrom the molten state to a solid state, (3) subjecting the polymer to anadditional solid-phase polymerization in the solid state, as desired,and (4) kneading the solid-state polymer in a molten state with theapplication of heat thereto.

BEST MODE FOR CARRYING OUT THE INVENTION

1. Cyclic Ester

For the cyclic ester used herein, bimolecular esters ofhydroxycarboxylic acids may be used. The hydroxy-carboxylic acids, forinstance, include glycolic acid, L-lactic acid, D-lactic acid,α-hydroxybutyric acid, α-hydroxyisobutyric acid, α-hydroxyvaleric acid,α-hydroxycaproic acid, α-hydroxyisocaproic acid, α-hydroxyheptanoicacid, α-hydroxyoctanoic acid, α-hydroxydecanoic acid, α-hydroxymyristicacid and α-hydroxystearic acid, which may or may not have beensubstituted by alkyl groups.

Of the cyclic esters, preference is given to glycolide that is abimolecular cyclic ester of glycolic acid as well as L-lactide andD-lactide that are bimolecular cyclic esters of lactic acid, althoughthe glycolide is most preferred. The ring-opening polymerization ofglycolide gives polyglycolic acid, and the ring-opening polymerizationof lactide gives polylactic acid. The glycolide and lactide may becopolymerized together.

Generally but not exclusively, the glycolide may be produced by thethermal depolymerization of glycolic acid oligomers. For instance, theglycolic acid oligomers may be depolymerized by such a solutiondepolymerization process as set forth in U.S. Pat. No. 2,668,162, such asolid-phase depolymerization process as set forth in JP-A 2000-119269,and such a solution depolymerization process as described in JP-A09-328481. Glycolide obtained as cyclic condensates of chloroacetic acidsalts as reported by K. Chujo et al. “Die Makromolekulare Cheme”,100(1967), pp. 262–266, too, may be used.

Glycolide, and lactide may be copolymerized with other comonomers which,by way of example, include cyclic monomers such as ethylene oxalate(i.e., 1,4-dioxane-2,3-dione), lactones (e.g., β-propiolactone,β-butyrolactone, pivalolactone, γ-butyrolactone, δ-valerolactone,β-methyl-δ-valerolactone, and ε-caprolactone), trimethylene carbonateand 1,3-dioxane; hydroxycarboxylic acids such as lactic acid,3-hydroxypropanoic acid, 3-hydroxybutanoic acid, 4-hydroxybutanoic acidand 6-hydroxycaproic acid as well as alkyl esters thereof; andsubstantially equimolar mixtures of aliphatic diols such as ethyleneglycol and 1,4-butandiol and aliphatic dicarboxylic acids such assuccinic acid and adipic acid or alkyl esters thereof. These comonomersmay be used in combination of two or more.

Particularly preferred among those comonomers are cyclic compounds suchas lactones and trimethylene carbonate; and hydroxycarboxylic acids suchas lactic acid and glycolic acid because they are so sensitive tocopolymerization that copolymers having excellent physical propertiescan be easily obtained.

The comonomer(s) is used in an amount of usually up to 45% by weight,preferably up to 30% by weight, and more preferably up to 10% by weightof all the charged monomers. By copolymerization, it is possible toobtain ring-opened copolymers having the desired physical properties.For instance, by the ring-opening polymerization of glycolide with othermonomer(s), it is possible to lower the melting point and, hence,processing temperature of polyglycolic acid, and control the rate ofcrystallization of polyglycolic acid, thereby improving itsprocessability on extrusion or elongation.

2. Polyhydroxycarboxylic Acid

To control the rate of biodegradation, the polyhydroxycarboxylic acidsof the present invention, for instance, polyglycolic acid, polylacticacid and glycolide/lactide copolymers should essentially have aweight-average molecular weight (Mw) in the range of 10,000 to 1,000,000and a molecular weight distribution (also called the degree ofmultidispersion) in the range of 1.0 to 2.5 as expressed by theweight-average molecular weight-to-number-average molecular weight(Mw/Mn).

If the weight-average molecular weight (Mw) of the polyhydroxycarboxylicacid of the present invention is within the range of 10,000 to1,000,000, satisfactory melt moldability and mechanical strength canthen be achieved, and the rate of biodegradation of thepolyhydroxycarboxylic acid can be controlled by regulating itsweight-average molecular weight. The weight-average molecular weightshould be within the range of preferably 20,000 to 800,000 and morepreferably 30,000 to 600,000. In most cases, satisfactory physicalproperties are obtainable within the range of 50,000 to 500,000. Too lowa weight-average molecular weight causes molded articles to becomebrittle and too high a weight-average molecular weight makes meltmolding difficult.

By limiting the molecular weight distribution of thepolyhydroxycarboxylic acid of the present invention to within the rangeof 1.0 to 2.5, the amount of a polymer component (low-molecular-weightportion) susceptible to premature biodegradation can be reduced tocontrol the rate of biodegradation of the polyhydroxycarboxylic acid.Too large a molecular weight distribution makes the rate ofbiodegradation of the polyhydroxycarboxylic acid unlikely to depend onthe weight-average molecular weight (or melt or solution viscosity)thereof. This molecular weight distribution should be in the range ofpreferably 1.3 to 2.4, and more preferably 1.5 to 2.3.

The rate of biodegradation may be controlled by setting theweight-average molecular weight within the aforesaid range andregulating the molecular weight distribution in the aforesaid range.More specifically, when a molded article comprisingpolyhydroxycarboxylic acid is disintegrated in the ground, the rate ofdisintegration (the rate of biodegradation) can be retarded. Too large amolecular weight distribution causes the rate of biodegradation tobecome fast, making the rate of biodegradation less likely to depend onthe molecular weight, even when the weight-average molecular weight isincreased. A narrow molecular weight distribution also makes it possibleto control the rate of biodegradation by the weight-average molecularweight.

When formed into a sheet by press molding and crystallization, thepolyhydroxycarboxylic acid of the present invention has a yellownessindex (YI) of as low as 40 or less or it is considerably reduced interms of coloration. This yellowness index should be preferably up to35, and more preferably up to 30. In most cases, the yellowness indexmay be reduced down to up to 25, and preferably up to 20. The yellownessindex should be usually at least 5 or, in most cases, at least 8although it should preferably be reduced as much as possible. When thepolyhydroxycarboxylic acid has too high a yellowness index, it gives amolded article of diminished commercial value because of being brownedor otherwise considerably colored. In addition, it is difficult to colorthe polyhydroxycarboxylic acid to the desired tone using coloringagents. It has been reported that considerably colored articles offerhygienic problems in the fields of food packaging material and medicalappliances.

The polyhydroxycarboxylic acid of the present invention shouldpreferably be excellent in terms of melt stability on melt molding. Morespecifically, the polyhydroxycarboxylic acid of the present inventionshould preferably have much more improved melt stability as expressed bythe retention of melt viscosity of at least 40%, which is defined by theproportion of the viscosity (η₆₀) measured after a 60-minute retentionat 250° C. to the initial viscosity (η₀) measured after a 5-minutepreheating at 250° C.: (η₆₀/η₀)×100.

In view of melt stability, the polyhydroxycarboxylic acid of the presentinvention should also preferably be such that when heated from 50° C. ata heating rate of 2° C./min. in a nitrogen stream at a flow rate of 10ml/min., the temperature at which the per cent loss from weight at 50°C. becomes 1% is 200° C. or higher.

3. Polyglycolic Acid Having Melt Stability

The polyhydroxycarboxylic acid of the present invention shouldpreferably be a polyglycolic acid excellent in melt stability. Thepolyglycolic acid having melt stability according to the presentinvention is now explained at great length.

The present polyglycolic acid having melt stability is not onlyexcellent in melt stability on melt molding but less colored as well.The melt stability may be objectively evaluated by the retention of meltviscosity, which is defined by the proportion of the viscosity (η₆₀)measured after a 60-minute retention at 250° C. to the initial viscosity(η₀) measured after a 5-minute preheating at 250° C.: (η₆₀/η₀)×100.

The present polyglycolic acid having melt stability can have a retentionof melt viscosity of at least 40% in general, at least 50% in manycases, and at least 60% in particular. The retention of melt viscosityis usually at least 80%, and at least 75% in most cases, although itshould preferably be as high as possible.

When the polyglycolic acid is molded by general melt molding processessuch as extrusion molding, the smaller the change in its retention ofmelt viscosity, the more stably the molding can be carried out. When theretention of melt viscosity of the polyglycolic acid is too low, forinstance, it is difficult to provide stable molding because there arefluctuations of torque on extrusion and breaks in sheets or films thatare being extruded. When the polyhydroxycarboxylic acid used has too lowmelt stability, much volatile components are generated during extrusion,often depositing onto members such as rolls.

When formed into sheets by press molding and crystallization, thepresent polyglycolic acid having melt stability has a yellowness index(YI) of as low as 40 or less or it is considerably reduced in terms ofcoloration.

The present polyglycolic acid having melt stability should preferably besuch that when heated from 50° C. at a heating rate of 2° C./min. in anitrogen stream at a flow rate of 10 ml/min., the temperature at whichthe per cent loss from weight at 50° C. becomes 1% is 200° C. or higher.As the temperature at which the per cent loss in weight becomes 1% islower than 200° C., much volatile gases are generated during meltmolding, depositing onto the resulting article to cause damage on itsoutside shape or depositing onto each part of the molding machine tocause contamination thereof. This temperature should be preferably 210°C. or higher, and more preferably 220° C. or higher, although it shouldpreferably be as high as possible. However, the upper-limit temperatureis usually 245° C. or lower, and often 240° C. or lower.

The present polyglycolic acid having melt stability should preferablyhave a melt viscosity in the range of 10 to 100,000 Pa·s as measured ata temperature of 240° C. and a shear rate of 122/sec. This meltviscosity should be in the range of more preferably 50 to 20,000 Pa·s,and even more preferably 100 to 10,000 Pa·s. When the polyglycolic acidhaving too low a melt viscosity is molded into an article, the articletends to become brittle due to decreased mechanical strength. Too high amelt viscosity renders it difficult to subject the polyglycolic acid tomelt molding.

In view of melt moldability and control of biodegradation, the presentpolyglycolic acid having melt stability should preferably have aweight-average molecular weight (Mw) in the range of 10,000 to 1,000,000and a molecular weight distribution (i.e., the degree ofmultidispersion) in the range of 1.0 to 2.5 as expressed by theweight-average molecular weight-to-number-average molecular weight ratio(Mw/Mn).

The present polyglycolic acid having melt stability should preferably beprovided in the form of pellets having a length of 1 to 10 mm and athickness of 1 to 10 mm. In consideration of shape, the pellet iseffective for carrying out molding in stable fashions. As pellet size(length and thickness) becomes smaller than that in the aforesaid range,the resin becomes vulnerable to electrostatic adherence to a moldingmachine during molding. Too large size results not only intime-consuming melting but also in the need of application of excessivethermal hysteresis.

4. Polyhydroxycarboxylic Acid Production Process

Polyhydroxycarboxylic acids may be produced by the ring-openingpolymerization (inclusive of ring-opening copolyermization) of suchcyclic esters as mentioned above. Usually, the polyhydroxycarboxylicacids are obtained by the ring-opening polymerization of cyclic estersin a bulk form.

To permit the weight-average molecular weight (Mw) and molecular weightdistribution (Mw/Mn) of a specific polyhydroxycarboxylic acid to comewithin the given ranges, for instance, it is of importance topredetermine (i) the type and amount of a polymerization catalyst, (ii)the type and amount of a molecular weight modifier, (iii) polymerizationconditions such as polymerizers, polymerization temperature andpolymerization time and (iv) post-polymerization treatments, and howthese factors are combined is also of importance.

To control the molecular weight of a polyhydroxycarboxylic acid productwhile the coloration of the product is reduced, it is preferable to usea process comprising a first ring-opening polymerization step (or thefirst polymerization step) and an additional polymerization step (or thesecond polymerization step) that is carried out for 1 to 50 hours at atemperature 10 to 50° C. lower than the first polymerizationtemperature.

By way of example but not by way of limitation, the polymerizationcatalysts used herein include tin compounds such as tin halides (e.g.,tin dichloride and tin tetrachloride) and organic carboxylic acid tin(e.g., tin octoate); titanium compounds such as alkoxytitanate; aluminumcompounds such as alkoxyaluminum; zirconium compounds such as zirconiumacetylacetone; and antimony halides. The amount of the polymerizationcatalyst used is, for example, in rage of preferably 1 to 1,000 ppm, andmore preferably 3 to 300 ppm on a weight ratio basis relative to thecyclic ester.

For control of the weight-average molecular weight, higher alcohols suchas lauryl alcohol may be added as the molecular weight modifier. For thepurpose of improving physical properties, it is acceptable to addpolyhydric alcohols such as glycerin.

For the reactor used, an appropriate selection may be made from variouspolymerization vessels such as those of the extruder type, the uprighttype having a paddle blade, the upright type having a helical ribbonblade, the horizontal extruder or kneader type, the ampoule type and thetubular type.

The temperature for the ring-opening (first) polymerization may bedetermined in the range of 120° C. to 250° C. The polymerizationtemperature should be in the range of preferably 130 to 240° C., morepreferably 140 to 230° C., and even more preferably 150 to 225° C. Attoo low polymerization temperatures the molecular weight distribution ofthe polymer product tends to become wide whereas at too highpolymerization temperatures the polymer product is sensitive to thermaldecomposition.

The time for the ring-opening (first) polymerization may be chosen fromthe range of 3 minutes to 50 hours. To keep the polymer product fromcoloration, it is preferable to shorten the polymerization time as thepolymerization temperature becomes high. To make the molecular weightdistribution sharp while the coloration of the polymer product isreduced, it is preferable to use polymerization conditions comprising arelatively high polymerization temperature and a relatively shortpolymerization time. The polymerization time is preferably in the rangeof 5 minutes to 30 hours. Too short a polymerization time makes thepolymerization less likely to proceed, whereas too long renders thepolymer product vulnerable to thermal decomposition.

The additional (second) polymerization is carried out at apolymerization temperature 10 to 50° C. lower than that for theaforesaid ring-opening (first) polymerization for a polymerization timeof 1 to 50 hours. The temperature for the additional polymerizationshould be lower than the first polymerization temperature by preferably15 to 48° C., and more preferably 20 to 45° C. A relatively long time ofpreferably 1.5 to 30 hours, more preferably 2 to 20 hours, and even morepreferably 3 to 15 hours should be applied to the additionalpolymerization. It is then desired that as the temperature for theadditional polymerization becomes low, the polymerization time becomeslong.

So far, ring-opening polymerization has been carried out at relativelylow temperatures so as to avoid thermal decomposition or coloration ofpolymer products. However, lower polymerization temperatures makepolymer products susceptible to crystallization and solidificationduring polymerization reactions, and so the polymerization reactionstend to become inhomogeneous with the result that polyhydroxycarboxylicacids having a wide molecular weight distribution yield. Elevatedpolymerization temperatures, on the other hand, make the molecularweight distribution of polymer products likely to become sharp. Even inthis case, however, it is desired to place the amount of catalysts andthe type and amount of molecular weight modifiers under control.

When the ring-opening polymerization or the first polymerization iscarried out at relatively low temperatures, it is preferred that afterthe completion of the polymerization reaction, the temperature of thepolymerization system be brought up to 220 to 250° C. or the polymerproduct be hot kneaded for the purpose of reducing low-molecular-weightmatters, thereby making the molecular weight distribution of the polymerproduct sharp.

5. Process for the Production of Polyglycolic Acid Having Melt Stability

For the production of polyglycolic acid particularly excellent in meltstability among the polyhydroxycarboxylic acids of the presentinvention, it is preferable to make use of the following productionprocess.

The polyglycolic acid of the present invention may be produced by thering-opening polymerization of glycolide that is a bimolecular cyclicester of glycolic acid according to the following reaction scheme:

To produce the polyglycolic acid having excellent melt stability, thering-opening polymerization of glycolide is carried out through a seriesof steps as mentioned below.

At step (1) glycolide is subjected to ring-opening polymerization in amolten state,

at step (2) the resulting polymer is converted from the molten state toa solid state,

at step (3) the polymer is further subjected to solid-phasepolymerization in the solid state as desired, and

at step (4) the solid-state polymer is hot kneaded with the applicationof heat thereto.

In accordance with the production process of the present invention, thering-opening polymerization of glycolide is first carried out in amolten state, and the resulting polymer is hot kneaded after conversionto a solid state. Alternatively, after the conversion to the solidstate, the polymer is further subjected to solid-phase polymerization,whereupon the polymer is hot kneaded. By use of such a productionprocess it is possible to produce a polymer having a narrow molecularweight distribution and a high retention of melt viscosity, because atthe aforesaid step (1) the ring-opening polymerization conditions areregulated to prevent coloration of the polymer product and at theaforesaid step (4) the hot kneading ensures that thermal treatment canbe carried out in a uniform molten state for a short period of timewhile any increase in the yellowness index is reduced.

In the aforesaid step (1) glycolide is subjected to ring-openingpolymerization at a temperature of 120° C. to 250° C. in the presence ofa small amount of the polymerization catalyst. Generally but notexclusively, the glycolide may be obtained by the thermaldepolymerization of glycolic acid oligomers.

By sole use of glycolide it is possible to obtain polyglycolic acid in ahomopolymer form. If the glycolide is used in combination with othercomonomer(s), it is then possible to obtain a copolymer(s) ofpolyglycolic acid. For the comonomer(s), use may be made of thosealready mentioned.

Preferred among the comonomers are cyclic compounds such as lactide,caprolactone and trimethylene carbonate; and hydroxycarboxylic acidssuch as lactic acid and glycolic acid because they are well sensitive tocopolymerization so that copolymers having excellent physical propertiescan be obtained. The comonomer(s) is used in an amount of usually up to45% by weight, preferably up to 30% by weight, and more preferably up to10% by weight of all the charged monomers. By copolymerization it ispossible to lower the melting point and, hence, processing temperatureof the polyglycolic acid, and control of the rate of crystallization ofthe polyglycolic acid, thereby improving its processability on extrusionor elongation.

The polymerization catalysts used herein, for instance, include theaforesaid tin compounds, titanium compounds, aluminum compounds,zirconium compounds, and antimony halides; however, the polymerizationcatalysts according to the present invention are not limited thereto.For adjustment of the weight-average molecular weight, higher alcoholssuch as lauryl alcohols may be added as the molecular weight modifier,and for the purpose of improving physical properties, polyhydricalcohols such as glycerin may be added.

For the reactor herein used, an appropriate selection may be made fromvarious polymerization vessels such as those of the extruder type, theupright type having a paddle blade, the upright type having a helicalribbon blade, the horizontal extruder or kneader type, the ampoule typeand the tubular type.

Depending on purposes, the polymerization temperature may be selectedfrom the range of 120° C. that is a substantial polymerization starttemperature to 250° C. The polymerization temperature should be in therange of preferably 130 to 240° C., more preferably 140 to 230° C., andeven more preferably 150 to 225° C. Too high a polymerizationtemperature makes the resulting polymer vulnerable to thermaldecomposition. The polymerization time is in the range of 3 minutes to20 hours, and preferably 5 minutes to 18 hours. Too short apolymerization time renders polymerization less likely to proceedsufficiently, and too long renders the resulting polymer susceptible tocoloration.

The optimum polymerization time may be chosen depending on thepolymerization temperature, polymerization catalyst, etc. When thereaction takes long at a polymerization temperature exceeding 225° C.,the resulting polymer is sensitive to coloration; that is, it ispreferable to make the reaction time short. At a polymerizationtemperature exceeding 225° C., the reaction time should be in the rangeof 3 to 20 minutes, and preferably 5 to 10 minutes. Preferably in mostcases, the ring-opening polymerization at step (1) should be carried outat a temperature of 225° C. or lower in the molten state.

At the aforesaid step (2), the polymer generated at step (1) isconverted from the molten state to the solid state. The conversion tothe solid state, for instance, may be carried out by (i) cooling thepolymer down from the polymerization temperature at which the polymer isin the molten state, (ii) precipitating or crystallizing and solidifyingthe polymer by carrying out the molten-state polymerization at atemperature lower than the melting point of the final polymer or (iii)adding a nucleating agent (such as talc, clay and titanium oxide) to thepolymer.

At step (3) the polymer is further subjected to solid-phasepolymerization as desired. The process, wherein the glycolide issubjected to ring-opening polymerization in the molten state, and theproduct is concerted to the solid state, subjected to solid-phasepolymerization and then hot kneaded, is effective to increase theretention of melt viscosity. Although any detailed reason has yet to beclarified, one possible explanation could be that the solid-phasepolymerization has an effect on making the polymerization degree profileof the polymer narrow. The solid-phase polymerization is carried out ata temperature at which the polymer is maintained in the solid state. Thesolid-phase polymerization temperature is in the range of usually 120°C. to less than 220° C. and preferably 140 to 200° C., and thesolid-phase polymerization time is in the range of usually 0.1 to 20hours and preferably 1 to 15 hours.

At the aforesaid step (4), the solid-state polymer is hot kneaded-withthe application of heat thereto. By hot kneading the polymer obtained byring-opening polymerization, it is possible to improve the meltstability of the polymer while any increase in the yellowness index ofthe polymer is reduced. Hot kneading may be carried out by any suitablemeans; however, it is preferable to make use of rolls, kneaders,extruders, etc. Particular preference is given to a biaxial kneader orextruder that ensures effective kneading. Although depending on theintended purpose, the object of the present invention may be achieved bycarrying out hot kneading two or more times.

The hot-kneading conditions should preferably be set such that the resintemperature be in the range of preferably 220 to 250° C., and morepreferably 225 to 245° C. When the resin temperature is too low duringhot kneading, no sufficient kneading is achievable with the result thatany improvement in melt stability is difficult to achieve. Too highresin temperatures make the polymer vulnerable to coloration. At thehot-kneading step, heat stabilizers may be added to the polymer.

As heat stabilizers, are preferred heavy metal deactivators, phosphateshaving a pentaerythritol skeleton structure, phosphorus compounds havingat least one hydroxyl group and at least one long-chain alkyl estergroup, metal carbonates, etc. These compounds may be used either singlyor in any combination thereof.

It has been found that many of phosphorus compounds such as phosphateantioxidants rather exhibit an effect to inhibit the melt stability ofpolyglycolic acid. On the other hand, the phosphates having apentaerythritol skeleton structure represented by the following formula(I):

exhibit an effect to specifically improve the melt stability of thepolyglycolic acid.

Specific examples of such phosphates having the pentaerythritol skeletonstructure include cyclicneopentanetetraylbis(2,6-di-tert-butyl-4-methyl-phenyl)phosphiterepresented by the formula (1):

cyclic neopentanetetraylbis(2,6-di-tert-butylphenyl)-phosphiterepresented by the formula (2):

a phosphite antioxidant represented by the formula (3):

and a phosphite antioxidant represented by the formula (4):

Among these, cyclicneopentanetetraylbis(2,6-di-tert-butyl-4-methylphenyl)phosphiterepresented by the formula (1) is particularly preferably because it hasan effect to markedly enhance the temperature at 3%-weight loss onheating of the polyglycolic acid even by the addition in a small amount.

Among the phosphorus compounds, are preferred phosphorus compoundshaving at least one hydroxyl group and at least one long-chain alkylester group represented by the formula (II):

The number of carbon atoms in the long-chain alkyl is preferably withina range of 8 to 24. Specific examples of such phosphorus compoundsinclude mono- or di-stearyl acid phosphate represented by the formula(5):

-   -   n=1 or 2

Example of the heavy metal deactivators include2-hydroxy-N-1H-1,2,4-triazol-3-yl-benzamide represented by the formula(6):

and bis[2-(2-hydroxybenzoyl)hydrazin]dodecanediacid represented by theformula (7):

Examples of the metal carbonates include calcium carbonate and strontiumcarbonate.

A proportion of these heat stabilizer incorporated is generally 0.001 to5 parts by weight, preferably 0.003 to 3 parts by weight, morepreferably 0.005 to 1 part by weight per 100 parts by weight of thecrystalline polyglycolic acid. The heat stabilizer is preferably thathaving an effect to improve the melt stability even by the addition inan extremely small amount. If the amount of the heat stabilizerincorporated is too great, the effect is saturated, or there is apossibility that the transparency of the resulting polyglycolic acidcomposition may be impaired.

The hot-kneading time is in the range of usually 1 to 20 minutes,preferably 3 to 15 minutes, and more preferably 5 to 10 minutes. Tooshort a hot-kneading time makes less contributions to melt stabilityimprovements whereas too long makes the polymer vulnerable tocoloration.

After the completion of hot kneading, it is preferable to collect thepolyglycolic acid in pellet forms, because there is no variation inextrusion due to the properties of powders during molding. Preferablepellet shape has already been explained.

By producing the polyglycolic acid through the aforesaid steps (1) to(4), it is possible to obtain polyglycolic acids improved in terms ofmelt stability, which has (I) a retention of melt viscosity of at least40% as defined by the proportion of the viscosity (η60) measured after a60-minute retention at 250° C. to the initial viscosity (η₀) measuredafter a 5-minute preheating at 250° C. (η₆₀/η₀×100), and/or (II) ayellowness index (YI) of up to 40 as measured with a sheet obtained bypress molding and crystallization of the polyglycolic acid.

According to the aforesaid production process, it is also possible toobtain (III) a polyglycolic acid designed such that when heated from 50°C. at a heating rate of 2° C./min. in a nitrogen stream at a flow rateof 10 ml/min., the temperature at which the per cent loss from weight at50° C. becomes 1% is 200° C. or higher, (IV) a polyglycolic acid havinga melt viscosity in the range of 10 to 100,000 Pa·s as measured at atemperature of 240° C. and a shear rate of 122/second, and (V) apolyglycolic acid having a molecular weight distribution in the range of1.0 to 2.5 as expressed by the weight-average molecularweight-to-number-average molecular weight ratio (Mw/Mn).

EXAMPLES

The present invention is now explained more specifically with referenceto inventive and comparative examples. The physical properties, etc.referred to below are measured as follows.

(1) Weight-Average Molecular Weight & Molecular Weight Distribution

Using a gel permeation chromatography (GPC) analyzer, the weight-averagemolecular weight (Mw) and molecular weight distribution (Mw/Mn) aremeasured under the following conditions.

Sodium trifluoroacetate (Kanto Chemical Industries, Ltd.) was added toand dissolved in hexafluoroisopropanol (a product made by Central GlassCo., Ltd. and distilled for use) to prepare a 5 mM solvent of sodiumtrifluoroacetate (A).

Solvent (A) is flowed through a column (HFIP-LG+HFIP-806M×2 made bySHODEX) at 40° C. at a flow rate of 1 ml/min. to prepare a 10 mlsolution with 10 mg of each of polymethyl methacrylates having fiveknown molecular weights of 827,000, 101,000, 34,000, 10,000 and 2,000(products made by POLYMER LABORATORIES Ltd.). Of each solution, 100 μlwas passed through the column to detect the refractive index (RI),thereby finding a detection peak time. The detection peak times andmolecular weights of five standard specimens are plotted to preparecalibration curves for molecular weights.

Then, solvent (A) was added to 10 mg of each specimen to prepare a 10 mlsolution. Of the solution, 100 μl was passed through the column to findweight-average molecular weight (Mw), number-average molecular weight(Mn) and molecular weight distribution (Mw/Mn) from the resultingelution curve. For calculation, C-R4AGPC Program Ver1.2, made byShimadzu Corporation, was used.

2. Melt Viscosity

Ten (10) grams of polyhydroxycarboxylic acid sandwiched between aluminumsheets are placed on a press machine preheated to 240° C. After a30-second preheating, the acid is pressed at 5 Mpa for 15 seconds, afterwhich it is rapidly cooled to make a sheet. The thus obtained amorphoussheet is heated at 150° C. in an oven for 30 minutes forcrystallization. The obtained crystallized sheet is cut out into arectangular piece of 5 mm in width and 50 to 75 mm in length to make amelt viscosity-measuring sample. The weight of the meltviscosity-measuring sample is 7 grams. This sample is put into acylinder having an inside diameter of 9.55 mm in Capirograph 3C made byToyo Seiki Co., Ltd., said cylinder being set at 240° C. Then, thesample is preheated for 5 minutes, after which the resin is extruded outof a die of 1 mm in inside diameter and 10 mm in length at a shear rateof 122/second, and the melt viscosity (Pa·s) of the sample is found fromthe then stress.

(3) Yellowness Index (YI)

Ten (10) grams of polyhydroxycarboxylic acid sandwiched between aluminumsheets are placed on a press machine preheated to 240° C. After a30-second preheating, the acid is pressed at 5 Mpa for 15 seconds, afterwhich it is rapidly cooled to make a sheet. The thus obtained amorphoussheet is heated at 150° C. in an oven for 30 minutes forcrystallization. Using Color Analyzer TC-1800MKII made by Tokyo DenshokuCo, Ltd., the yellowness index (YI) of the crystallized sheet isdetermined. Three measurements are obtained under the conditions offield of view of 2°, standard light C, and measurement of reflectedlight to calculate their average defining the yellowness index (YI) ofthe polyhydroxycarboxylic acid.

(4) Degradability in Soil

After heated and pressed at 240° C. for 30 seconds,polyhydroxycarboxylic acid was rapidly cooled to prepare a sheet. Thissheet was buried at a depth of 15 cm in a potato plot at a private housesite in the city of Iwaki, Fukushima Prefecture, Japan. Morespecifically, such sheets were horizontally placed in a 15-cm-deep holedug in the ground in a non-superimposed manner, and covered with earth.After the lapse of a given period, the ground was carefully turned up tocheck sheet shape according to the following criteria.

A: The sheet is kept in good shape.

B: The sheet shape disintegrates partly.

C: The sheet disintegrates.

(5) Retention of Melt Viscosity

Using RSDII made by Rheometrics Co., Ltd. in a nitrogen stream, 2 gramsof polyglycolic acid are set between parallel plates of ½ inch indiameter at a gap length of 1.5 mm. After a 5-minute preheating at 250°C., the initial viscosity (η₀; Pa·s) of polyglycolic acid is measured atan angular velocity of 10 rad/s. After a 60-minute retention at 250° C.,on the other hand, the viscosity (η₆₀; Pa·s) of polyglycolic acid ismeasured at an angular velocity of 10 rad/s. The retention of meltviscosity is calculated from the following equation: Retention of meltviscosity (%)=(η₆₀)/(η₀)×100

(6) Per Cent Loss in Weight

Using TG50 made by Metler Co., Ltd. in a nitrogen atmosphere whereinnitrogen prevails at a flow rate of 10 ml/min., polyglycolic acid isheated from 50° C. at a heating rate of 2° C./min. to measure the percent loss in weight. The temperature at which the weight (W₅₀) at 50° C.of polyglycolic acid shows a 1% loss is precisely read out. Herein thistemperature is used to define the temperature at which the per cent lossin weight of polyglycolic acid becomes 1%.

(7) Moldability

Polyglycolic acid is put into a uniaxial extruder having a cylinder of20 mm in inside diameter, over which a T-die of 200 mm in width ismounted, wherein the polyglycolic acid is hot extruded in a sheet form.Then, the sheet is wound around a cooling roll for sheet molding.Moldability was evaluated on the following criteria:

A: Stable extrusion molding is feasible over an extended period of time,and the molded sheet is transparent and substantially colorless.

B: Stable extrusion molding is feasible; however, the molded sheet showsa brown color.

C: Fluctuations in extrusion torque are observed during sheet extrusionmolding, and there are also breaks in sheets being extruded; stablemolding has difficulty.

Example 1

One hundred (100) grams of glycolide, 5 mg of tin tetrachloride and 50mg of lauryl alcohol were put into a glass test tube for a 3-hourpolymerization at 200° C. After the polymerization, an additional12-hour polymerization was carried out at 160° C. After the additionalpolymerization, the resulting polymer was cooled, then collected, thenpulverized and then washed with acetone. Following this, vacuum dryingat 30° C. gave a polymer product. The properties of the obtainedpolyglycolic acid are shown in Table 1 together with the results of soildegradability test.

Example 2

Polyglycolic acid was prepared as in Example 1 with the exception thatthe amount of lauryl alcohol was changed to 40 mg. The results are shownin Table 1.

Example 3

Polyglycolic acid was prepared as in Example 1 with the exception thatthe amount of lauryl alcohol was changed to 5 mg. The results are shownin Table 1.

Comparative Example 1

Polyglycolic acid was prepared as in Example 1 with the exception thatno additional polymerization was carried out. The results are shown inTable 1.

Comparative Example 2

Polyglycolic acid was prepared as in Example 3 with the exception thatno additional polymerization was carried out. The results are shown inTable 1.

Comparative Example 3

One hundred (100) grams of glycolide, 5 mg of tin tetrachloride and 5 mgof lauryl alcohol were put into a glass test tube for a 2-hourpolymerization at 240° C. After the polymerization, the resultingpolymer was cooled, then collected, then pulverized and then washed withacetone. Following this, vacuum drying at 30° C. gave a polymer product.The properties of the obtained polyglycolic acid are shown in Table 1together with the results of soil degradability test.

TABLE 1 Ring-Opening Polymerization of Glycolide Melt AdditionalViscosity Conditions Polymerization Pa · s Temp./Time Temp./Time (240°C., (° C./h) (° C./h) 122/s) Ex. 1 200° C./3 h 160° C./12 h 280 Ex. 2200° C./3 h 160° C./12 h 460 Ex. 3 200° C./3 h 160° C./12 h 2,150 Comp.Ex. 1 200° C./3 h — 320 Comp. Ex. 2 200° C./3 h — 2,200 Comp. Ex. 3 240°C./3 h — 2,300 Soil Degradability Test After Mw Mw/Mn YI 2 Weeks 4 Weeks8 Weeks Ex. 1 110,000 1.9 17 B C C Ex. 2 135,000 2.1 18 A B C Ex. 3238,000 1.7 16 A A C Comp. Ex. 1 117,000 2.8 15 C C C Comp. Ex. 2245,000 4.2 17 C C C Comp. Ex. 3 248,000 2.1 66 A A C Mw: weight-averagemolecular weight Mw/Mn: molecular weight distribution YI: yellownessindex

As can be seen from the results of Table 1, the polymer samples (Comp.Examples 1–2) having a wide molecular weight distribution exhibitpremature disintegration in the soil degradability test irrespective ofthe magnitude of their weight-average molecular weight. By contrast, thepolymer-samples (Inventive Examples 1–3) having a sharp molecular weightdistribution are not only reduced in terms of premature disintegrationin the ground but also have their rates of biodegradation controllableby the regulation of their weight-average molecular weight.

Example 4

One hundred (100) grams of glycolide and 4 mg of tin dichloride·2H₂Owere put into a glass test tube wherein they were stirred at 200° C. for1 hour and then left standing for 3 hours for ring-openingpolymerization. After the completion of the polymerization, theresulting polymer was cooled, then taken out, then pulverized, and thenwashed with acetone. The polymer was then vacuum-dried at 30° C. tocollect the polymer. Then, the polymer was put into Labo Plastomill madeby Toyo Seiki Co., Ltd., which was preset to 230° C., and melted andkneaded for 10 minutes.

The thus obtained polyglycolic acid was found to have a retention ofmelt viscosity of 59%, a crystallized sheet's yellowness index (YI) of27.2, a temperature of 225° C. at which the per cent loss in weightbecame 1%, a weight-average molecular weight (Mw) of 245,000, amolecular weight distribution (Mw/Mn) of 2.1, and a melt viscosity of500 Pa·s.

Ten (10) kg of polyglycolic acid obtained in the same manner were putinto a uniaxial extruder having a cylinder of 20 mm in inside diameter,over which a T-die of 200 mm in width was mounted. Through the extruderthe polymer was extruded in a sheet form, which was then wound around acooling roll to make a sheet. The obtained sheet was transparent andsubstantially colorless. Even after the lapse of 6 hours from the startof extrusion, stable molding was feasible. The results are tabulated inTable 2.

Example 5

LT-20 made by Toyo Seiki Co., Ltd. with a 5 mm-holed die was used at 15rpm and a preset temperature of 200 to 240° C. (resin temperature: 240°C.). Glycolide with 300 ppm of tin tetrachloride·5H₂O added thereto wasput from a hopper into the assembly for ring-opening polymerization. Astrand leaving the die was hot cut to obtain pellets of 6 mm in lengthand 3 mm in thickness. A dyed pellet was put from the hopper into theassembly to measure the period of time (residence time) until the dyedresin was distilled out. This residence time was found to be 7 minutes.Furthermore, the obtained polymer was put into Labo Plastomill made byToyo Seiki Co., Ltd., which was preset to 230° C., and melted andkneaded for 15 minutes.

The thus obtained polyglycolic acid was found to have a retention ofmelt viscosity of 41%, a crystallized sheet's yellowness index (YI) of16.5, a temperature of 220° C. at which the per cent loss in weightbecame 1%, a weight-average molecular weight (Mw) of 120,000, amolecular weight distribution (Mw/Mn) of 2.2, and a melt viscosity of300 Pa·s.

Ten (10) kg of polyglycolic acid obtained in the same manner were putinto a uniaxial extruder having a cylinder of 20 mm in inside diameter,over which a T-die of 200 mm in width was mounted. Through the extruderthe polymer was extruded in a sheet form, which was then wound around acooling roll to make a sheet. The obtained sheet was transparent andsubstantially colorless. Even after the lapse of 6 hours from the startof extrusion, stable molding was feasible. The results are tabulated inTable 2.

Example 6

One hundred (100) grams of glycolide and 4 mg of tin dichloride·2H₂Owere put into a glass test tube for a 2-hour ring-openingpolymerization-at 180° C. After the completion of the reaction, theresulting polymer was solidified. The solid-state polymer was leftstanding at 160° C. for 10 hours for an additional solid-phasepolymerization. After the completion of the polymerization, the polymerwas cooled, then taken out, then pulverized, and then washed withacetone. The polymer was then vacuum-dried at 30° C. to collect thepolymer. Then, the polymer was put into Labo Plastomill made by ToyoSeiki Co., Ltd., which was preset to 230° C., and melted and kneaded for10 minutes.

The thus obtained polyglycolic acid was found to have a retention ofmelt viscosity of 65%, a crystallized sheet's yellowness index (YI) of15.8, a temperature of 231° C. at which the per cent loss in weightbecomes 1%, a weight-average molecular weight (Mw) of 290,000, amolecular weight distribution (Mw/Mn) of 1.8, and a melt viscosity of800 Pa·s.

Ten (10) kg of polyglycolic acid obtained in the same manner were putinto a uniaxial extruder having a cylinder of 20 mm in inside diameter,over which a T-die of 200 mm in width was mounted. Through the extruderthe polymer was extruded in a sheet form, which was then wound around acooling roll to make a sheet. The obtained sheet was transparent andsubstantially colorless. Even after the lapse of 6 hours from the startof extrusion, stable molding was feasible. The results are tabulated inTable 2.

Example 7

One hundred (100) grams of glycolide and 5 mg of tin dichloride·2H₂Owere put into a glass test tube for a 4-hour ring-opening polymerizationat 180° C. After the completion of the reaction, the resulting polymerwas solidified. The polymer was cooled, then taken out, then pulverized,and then washed with acetone. The polymer was then vacuum-dried at 30°C. to collect the polymer. The obtained polymer was extruded throughLT-20 made by Toyo Seiki Co., Ltd. with a 5 mm-holed die operating at 30rpm and a preset temperature of 200 to 240° C. (resin temperature: 240°C.). A strand leaving the die was hot cut to obtain pellets of 6 mm inlength and 3 mm in thickness. A dyed pellet was put from the hopper intothe assembly to measure the period of time (residence time) until thedyed resin was distilled out. This residence time was found to be 5.5minutes.

The thus obtained polyglycolic acid was found to have a retention ofmelt viscosity of 61%, a crystallized sheet's yellowness index (YI) of10.2, a temperature of 230° C. at which the per cent loss in weightbecame 1%, a weight-average molecular weight (Mw) of 260,000, amolecular weight distribution (Mw/Mn) of 1.9, and a melt viscosity of780 Pa·s.

Ten (10) kg of polyglycolic acid obtained in the same manner were putinto a uniaxial extruder having a cylinder of 20 mm in inside diameter,over which a T-die of 200 mm in width was mounted. Through the extruderthe polymer was extruded in a sheet form, which was then wound around acooling roll to make a sheet. The obtained sheet was transparent andsubstantially colorless. Even after the lapse of 12 hours from the startof extrusion, stable molding was feasible. The results are tabulated inTable 2.

Example 8

Following Example 7, 100 grams of glycolide and 5 mg of tindichloride·2H₂o were charged in a glass test tube for a 4-hourring-opening polymerization at 180° C. One hundred (100) parts by weightof the collected polyglycolic acid were mixed with 0.03 part by weightof a heat stabilizer, i.e., a phosphate antioxidant represented by theaforesaid formula (4) (PEP-8 made by Asahi Denka Kogyo Co., Ltd.) toobtain a mixture, which was then fed into an extruder LT-20 to preparepellets as in Example 7. The results are tabulated in Table 2.

Example 9

One hundred (100) grams of glycolide and 3 mg of tin dichloride·2H₂Owere put into a glass test tube for a 24-hour ring-openingpolymerization at 170° C. After the completion of the polymerization,the polymer was cooled, then taken out, then pulverized, and then washedwith acetone. Then, the polymer product was put into Labo Plastomillmade by Toyo Seiki Co., Ltd., which was preset to 230° C., and meltedand kneaded for 10 minutes.

The thus obtained polyglycolic acid was found to have a retention ofmelt viscosity of 60%, a crystallized sheet's yellowness index (YI) of13.4, a temperature of 229° C. at which the per cent loss in weightbecomes 1%, a weight-average molecular weight (Mw) of 239,000, amolecular weight distribution (Mw/Mn) of 2.2, and a melt viscosity of770 Pa·s.

Ten (10) kg of polyglycolic acid obtained in the same manner were putinto a uniaxial extruder having a cylinder of 20 mm in inside diameter,over which a T-die of 200 mm in width was mounted. Through the extruderthe polymer was extruded in a sheet form, which was then wound around acooling roll to make a sheet. The obtained sheet was transparent andsubstantially colorless. Even after the lapse of 6 hours from the startof extrusion, stable molding was feasible. The results are tabulated inTable 2.

Comparative Example 4

One hundred (100) grams of glycolide and 5 mg of tin dichloride·2H₂Owere put into a reactor having a helical ribbon blade, wherein they werestirred at 230° C. for 2 hours for ring-opening polymerization. Afterthe completion of the reaction, the polymer was scraped out, and thewashed with acetone. Then, the polymer was vacuum-dried at 30° C. tocollect the polymer.

The thus obtained polyglycolic acid was found to have a retention ofmelt viscosity of 42%, a crystallized sheet's yellowness index (YI) of67.8, a temperature of 230° C. at which the per cent loss in weightbecame 1%, a weight-average molecular weight (Mw) of 250,000, amolecular weight distribution (Mw/Mn) of 2.2, and a melt viscosity of750 Pa·s.

Ten (10) kg of polyglycolic acid obtained in the same manner were putinto a uniaxial extruder having a cylinder of 20 mm in inside diameter,over which a T-die of 200 mm in width was mounted. Through the extruderthe polymer was extruded in a sheet form, which was then wound around acooling roll to make a sheet. The obtained sheet was brown. Althoughstable extrusion molding was feasible, yet the color tone of the polymerwas less than satisfactory. The results are tabulated in Table 2.

Comparative Example 5

One hundred (100) grams of glycolide and 5 mg of tin dichloride·2H₂Owere put into a glass test tube, wherein they were left standing at 180°C. for 4 hours for ring-opening polymerization. Upon the completion ofthe polymerization, the polymer was solidified. The polymer was cooled,then taken out, then pulverized, and then washed with acetone. Then, thepolymer was vacuum-dried at 30° C. to collect the polymer.

The thus obtained polyglycolic acid was found to have a retention ofmelt viscosity of 21%, a crystallized sheet's yellowness index (YI) of10.1, a temperature of 190° C. at which the per cent loss in weightbecame 1%, a weight-average molecular weight (Mw) of 250,000, amolecular weight distribution (Mw/Mn) of 2.7, and a melt viscosity of970 Pa·s.

Ten (10) kg of polyglycolic acid obtained in the same manner were putinto a uniaxial extruder having a cylinder of 20 mm in inside diameter,over which a T-die of 200 mm in width was mounted. Through the extruderthe polymer was extruded in a sheet form, which was then wound around acooling roll to make a sheet. The obtained sheet was transparent andsubstantially colorless. However, fluctuations of extrusion torque wereobserved with frequent breaks in sheets being extruded; any stablemolding was unfeasible. There were also large amounts of volatilecomponents generated during extrusion with deposition of the volatilecomponents onto the roll. The results are tabulated in Table 2.

TABLE 2 Polyglocolic Acid Melt Viscosity Weight-Average Molecular Pa · s(240° C., Molecular Weight Weight Distribution 122/s) (Mw) (Mw/Mn)Example 4 500 245,000 2.1 Example 5 300 120,000 2.2 Example 6 800290,000 1.8 Example 7 780 260,000 1.9 Example 8 800 260,000 1.9 Example9 770 239,000 2.2 Comp. Ex. 4 750 250,000 2.2 Comp. Ex. 5 970 250,0002.7 Properties Temperature At Which Retention of The Per Cent Loss InMelt Viscosity (%) YI Weight Became 1% (° C.) Example 4 59 27.2 225Example 5 41 16.5 220 Example 6 65 15.8 231 Example 7 61 10.2 230Example 8 82 9.5 233 Example 9 60 13.4 229 Comp. Ex. 4 42 67.8 230 Comp.Ex. 5 21 10.1 190 Soil Degradability Test After 2 Weeks 4 Weeks 8 WeeksMoldability Example 4 A B C A Example 5 B C C A Example 6 A A C AExample 7 A A C A Example 8 A A A A Example 9 A A C A Comp. Ex. 4 B C CB Comp. Ex. 5 C C C C

As can be seen from the results of Table 2, the present polyglycolicacids having melt stability (Inventive Examples 4 to 7) have a highretention of melt viscosity and a low yellowness index (YI) and are soimproved in moldability that transparent, substantially colorless sheetscan be obtained in a stable fashion. The present polyglycolic acidshaving melt stability (Inventive Examples 4 to 7) are also controllablein terms of biodegradability by the regulation of their weight-averagemolecular weight and molecular weight distribution.

By contrast, the polyglycolic acid that has not been hot kneaded afterthe ring-opening polymerization has a high yellowness index (YI) evenwhen the polymerization is carried out under such conditions as to givea narrow molecular weight distribution (low Mw/Mn) (Comparative Example4) with the results that molded articles assume a brown color. Also, thepolyglycolic acid that has not been hot kneaded after the ring-openingpolymerization has a very low retention of melt viscosity and a loweringof the temperature at which the per cent loss in weight becomes 1% andis poor in melt stability and moldability as well, even when thepolymerization is carried out such conditions as to make the yellownessindex (YI) low (Comparative Example 5). Further, the polyglycolic acidof Comparative Example 2 has difficulty in controlling the degree ofbiodegradability.

INDUSTRIAL APPLICABILITY

According to the present invention, there are providedpolyhydroxycarboxylic acids controlled in terms of the rate ofbiodegradation and reduced in terms of coloration. According to thepresent invention, there is also provided a polyhydroxycarboxylic acidensuring that its molded articles are of uniform quality with neitherany premature strength drop nor any premature deterioration of retentionof outside shape. According to the present invention, there are furtherprovided a less colored polyglycolic acid having excellent meltstability and its production process. In particular, the polyglycolicacid having melt stability according to the present invention is ofbiodegradability and improved in gas barrier properties, heatresistance, moldability, mechanical strength, etc.

The polyhydroxycarboxylic acids of the present invention are useful forvarious molded or otherwise formed articles such as sheets, films andfibers, composite materials (e.g., multilayer films or containers) andso on. The polyhydroxycarboxylic acids of the present invention, becausethey can have controlled rates of biodegradation, are easily fitted forintended use in various application fields.

1. A polyglycolic acid obtained by ring-opening polymerization ofglycolide or glycolide with other cyclic comonomer(s) in an amount of upto 10% by weight of all the charged monomers, which has the followingproperties (a) to (c): (a) a weight-average molecular weight Mw in therange of 10,000 to 1,000,000, (b) a molecular weight distribution in therange of 1.0 to 2.5 as represented by a weight-average molecularweight-to-number-average molecular weight ratio Mw/Mn, and (c) ayellowness index YI of up to 40 as measured using a sheet obtained bypress molding and crystallization of said polyglycolic acid.
 2. Thepolyglycolic acid according to claim 1, which has (d) a retention ofmelt viscosity of at least 40% as defined by a proportion of theviscosity (η₆₀) of said polyglycolic acid measured after a 60-minuteretention at 250° C. to the initial viscosity (η₀) of said polyglycolicacid measured after a 5-minute preheating at 250° C. ((η₆₀/η₀)×100). 3.The polyglycolic acid according to claim 1, wherein (e) when saidpolyglycolic acid is heated from 50° C. at a heating rate of 2° C. /min.in a nitrogen stream at a flow rate of 10 ml/min., a 1% weight loss fromthe polyglycolic acid weight at 50° C. occurs at a temperature not lessthan 200° C.
 4. The polyglycolic acid according to claim 1, which has(f) a melt viscosity in the range of 10 to 100,000 Pa·s as measured at atemperature of 240° C. and a shear rate of 122/second.
 5. A polyglycolicacid obtained by ring-opening polymerization of glycolide or glycolidewith other cyclic comonomer(s) in an amount of up to 10% by weight ofall the charged monomers and having melt stability, which has: (I) aretention of melt viscosity of at least 40% as defined by a proportionof the viscosity (η₆₀) of said polyglycolic acid measured after a60-minute retention at 250° C. to the initial viscosity (η₀) of saidpolyglycolic acid measured after a 5-minute preheating at 250° C.((η₆₀/η₀)×100), and (II) a yellowness index (YI) of 40 or less asmeasured using a sheet obtained by press molding and crystallization ofsaid polyglycolic acid.
 6. The polyglycolic acid having melt stabilityaccording to claim 5, wherein (III) when said polyglycolic acid isheated from 50° C. at a heating rate of 2° C. /min. in a nitrogen streamat a flow rate of 10 ml/min., a 1% loss from the polyglycolic acidweight at 50° C. occurs at a temperature not less than 200° C.
 7. Thepolyglycolic acid having melt stability according to claim 5, which has(LV) a melt viscosity in the range of 10 to 100,000 Pa·s as measured ata temperature of 240° C. and a shear rate of 122/second.
 8. Thepolyglycolic acid having melt stability according to claim 5, which has(V) a weight-average molecular weight (Mw) in the range of 10,000 to1,000,000 and a molecular weight distribution in the range of 1.0 to 2.5as represented by a weight-average molecular weight-to-number-averagemolecular weight ratio (Mw/Mn).
 9. A pellet of 1 to 10 mm in length and1 to 10 mm in thickness, which is formed of the polyglycolic acidaccording to claim
 5. 10. A polyhydroxycarboxylic acid productionprocess, comprising ring-opening polymerization of a cyclic ester at 120to 250° C. for 3 minutes to 50 hours, and an additional polymerizationthat is carried out at a temperature 10 to 50° C. lower than thetemperature for said ring-opening polymerization for 1 to 50 hours. 11.A polyhydroxycarboxylic acid production process, comprising ring-openingpolymerization of a cyclic ester at 120 to 250° C. for 3 minutes to 50hours, and an additional polymerization that is carried out at atemperature 10 to 50° C. lower than the temperature for saidring-opening polymerization for 1 to 50 hours, wherein apolyhydroxycarboxylic acid having: (a) a weight-average molecular weight(Mw) in the range of 10,000 to 1,000,000, (b) a molecular weightdistribution in the range of 1.0 to 2.5 as represented by aweight-average molecular weight-to-number-average molecular weight ratio(Mw/Mn), and (c) a yellowness index (YI) of up to 40 as measured using asheet obtained by press molding and crystallization of saidpolyhydroxycarboxylic acid is produced.
 12. A process for producing apolyglycolic acid having melt stability, comprising steps of: (1)subjecting glycolide or glycolide with other cyclic comonomer(s) in anamount of up to 10% by weight of all the charged monomers toring-opening polymerization in a molten state at a temperature of 140 to230° C., (2) converting a polymer product from the molten state to asolid state by (i) cooling the polymer down from the polymerizationtemperature at which the polymer is in the molten state, (ii)precipitating or crystallizing and solidifying the polymer by carryingout the molten-state polymerization at a temperature lower than themelting point of the final polymer, or (iii) adding a nucleating agentto the polymer, and (3) hot kneading the solid-state polymer productwith application of heat thereto at a temperature of 220 to 250° C. in atime range of 5 to 20 minutes.
 13. The production process according toclaim 12, wherein the hot-kneading of the polymer at step (3) is carriedout using a roll, a kneader or an extruder.
 14. The production processaccording to claim 12, wherein a polyglycolic acid having (I) aretention of melt viscosity of at least 40% as defined by a proportionof the viscosity (η₆₀) of said polyglycolic acid measured after a60-minute retention at 250° C. to the initial viscosity (η₀) of saidpolyglycolic acid measured after a 5-minute preheating at 250° C.((η₆₀/η₀)×100) is produced.
 15. The production process according toclaim 12, wherein a polyglycolic acid having (II) a yellowness index(YI) of 40 or less as measured using a sheet obtained by press moldingand crystallization of said polyglycolic acid is produced.
 16. Theproduction process according to claim 12, wherein a polyglycolic acidwherein (III) when said polyglycolic acid is heated from 50° C. at aheating rate of 2° C. /min. in a nitrogen stream at a flow rate of 10ml/min., a 1% loss from the polyglycolic acid weight at 50° C. occurs ata temperature not less than 200° C. is produced.
 17. The productionprocess according to claim 12, wherein a polyglycolic acid having (IV) amelt viscosity in the range of 10 to 100,000 Pa·s as measured at atemperature of 240° C. and a shear rate of 122/second is produced. 18.The production process according to claim 12, wherein a polyglycolicacid having (V) a weight-average molecular weight (Mw) in the range of10,000 to 1,000,000 and a molecular weight distribution in the range of1.0 to 2.5 as represented by a weight-average molecularweight-to-number-average molecular weight ratio (Mw/Mn) is produced. 19.The production process according to claim 12, which further comprises,between said steps (2) and (3), the following step: subjecting thepolymer product to solid-phase polymerization at a temperature of 120°C. to less than 220° C. in a time range of 0.1 to 20 hours.
 20. Theproduction process according to claim 12, wherein, at the hot kneadingstep (3), at least one heat stabilizer selected from the groupconsisting of metal activators, phosphates having a pentaerythritolskeleton structure, phosphorus compounds having at least one hydroxylgroup and at least one long-chain alkyl ester group, and metalcarbonates, is added to the solid-state polymer product.
 21. A processfor producing a polyglycolic acid having melt stability, comprisingsteps of: (1) subjecting glycolide or glycolide with other cycliccomonomer(s) in an amount of up to 10% by weight of all the chargedmonomers to ring-opening polymerization in a molten state at atemperature of 140 to 230° C., (2) converting a polymer product from themolten state to a solid state by (i) cooling the polymer down from thepolymerization temperature at which the polymer is in the molten state,(ii) precipitating or crystallizing and solidifying the polymer bycarrying out the molten-state polymerization at a temperature lower thanthe melting point of the final polymer, or (iii) adding a nucleatingagent to the polymer, (3) subjecting the polymer product to solid-phasepolymerization at a temperature of 120° C. to less than 220° C. in atime range of 0.1 to 20 hours, and (4) hot kneading the solid-statepolymer product with application of heat thereto at a temperature of 220to 250° C. in a time range of 5 to 20 minutes.
 22. A process forproducing a polyglycolic acid having melt stability, comprising stepsof: (1) subjecting glycolide or glycolide with other cyclic comonomer(s)in an amount of up to 10% by weight of all the charged monomers toring-opening polymerization in a molten state at a temperature of 140 to230° C., (2) converting a polymer product from the molten state to asolid state by (i) cooling the polymer down from the polymerizationtemperature at which the polymer is in the molten state, (ii)precipitating or crystallizing and solidifying the polymer by carryingout the molten-state polymerization at a temperature lower than themelting point of the final polymer, or (iii) adding a nucleating agentto the polymer, and (3) hot kneading the solid-state polymer productwith application of heat thereto at a temperature of 220 to 250° C. in atime range of 5 to 20 minutes, wherein, at the hot kneading step (3), atleast one heat stabilizer selected from the group consisting of metalactivators, phosphates having a pentaerythritol skeleton structure,phosphorus compounds having at least one hydroxyl group and at least onelong-chain alkyl ester group, and metal carbonates, is added to thesolid-state polymer product.